chore(nixos): re-organise packages

Remove packages that are either no longer used or have been moved into
per-project flakes that are used with direnv.

system/nixos/home-manager-desktop.nix

@@ -1,11 +1,6 @@
 { config, inputs, pkgs }:
 
 {
-  nixpkgs = {
-    config.allowUnfreePredicate = pkg:
-      builtins.elem (inputs.lib.getName pkg) [ "postman" ];
-  };
-
   services.swayidle = {
     enable = true;
 

system/nixos/home-manager.nix

@@ -15,7 +15,6 @@ in
     pkgs.pamixer
     pkgs.pass
     pkgs.pinentry
-    pkgs.postman
     pkgs.slack
     pkgs.via
     pkgs.vial